Ionization smoke detectors are of various types, the commonest comprising a system of electrodes whose respective potentials are dependent upon the flow of ionization current in separate chambers, and electronic circuitry interconnected with the electrodes for deriving an electrical signal in response to a change in the configuration of electrode potentials caused by the entry of smoke into one of the chambers. A suitable alarm device such as a piezoelectric horn is normally connected to output terminals of the electronic circuitry. An LED connected to the circuitry may also provide a visual indication of the alarm condition.
In one such type of smoke detector, to which the present invention relates, the electrode system comprises an inner electrode consisting of or supporting a radioactive material or otherwise providing a source of ionizing radiation, an outer electrode providing openings which allow the passage of smoke therethrough, and an auxiliary electrode positioned so as to define with the inner and outer electrodes, respectively, a first and a second ionization chamber disposed one within the other, the auxiliary electrode having a hole which is capable of passing radioactive rays from the inner electrode to the outer chamber so as to produce ionization simultaneously in both chambers.
Known smoke detectors of this type have the disadvantage that their structures tend to be large and often complicated, and they do not readily lend themselves to automatic assembly techniques during manufacture.
It is an object of the present invention to provide an improved smoke detector of this type which, besides having the desirable features of the known detectors has the added advantages of the simplicity of construction and ease of assembly during manufacture.